Two-time Grand Slam champion Li Na is out of next month's US Open because of a knee injury.

The Chinese player, currently ranked second in the world, has been out of action since losing to Barbora Zahlavova-Strycova in the third round at Wimbledon.

She confirmed the knee had been bothering her since March and had been advised by her medical team to take a complete rest, ruling her out until at least the end of August.

The reigning Australian Open champion's aim is to return for the Wuhan Open in her homeland in the middle of September.

"I wanted to let all of my great fans know that unfortunately I am going to have to withdraw from the upcoming WTA events in Montreal and Cincinnati, as well as the US Open," Li said on her Facebook page.

"Since March, I have been struggling with my knee and it is just not where I need it to be in order to play at the highest level.

"My medical team has advised me that I need to take some time off to rest my knee so it heals. I look forward to getting back on the court this fall in Wuhan and Beijing."

Li's enforced absence means French Open runner-up Simona Halep will climb to second in the WTA rankings, a career high, after next week's Rogers Cup event in Montreal.